Nuclear magnetic resonance (NMR) refers to the response of atomic nuclei to magnetic fields. It is applicable to nuclei having an odd number of protons or neutrons, or both. In many applications, the nuclei is that of hydrogen (a proton), where the hydrogen is part of the water molecule.
A static magnetic field is applied to the sample of interest, which causes a precession of the nuclei at the Larmor frequency, proportional to the strength of the applied static magnetic field. The applied static magnetic field has a non-zero spatial gradient, so that the Larmor frequency of a nucleus is a function of its position in the sample of interest. The macroscopic magnetization is parallel with the direction of the static magnetic field.
Applying an oscillating magnetic field perpendicular to the static magnetic field and at a frequency equal to the Larmor frequency tips the magnetization. The tip angle is proportional to the product of the amplitude of the oscillating magnetic field with the time over which it is applied. The nuclei with the Larmor frequency precess in phase with one another.
A 90° pulse refers to a pulsed oscillating magnetic field that tips the magnetization to a direction along a plane transverse to the direction of the static magnetic field. After the 90° pulse, the nuclei population begins to dephase, that is, they lose their phase coherency. This decreases the net magnetization, which may be detected by a receiver coil. The measured decay is called the free induction decay.
In spin-echo detection, a sequence of 180° pulses (oscillating magnetic field pulses that change the tip angle by 180°) follows the 90° pulse. The first 180° pulse reverses the dephasing of the magnetization among the nuclei population, so that after some period of time the nuclei tend to be in phase, and a spin-echo signal is generated that is detectable in a receiver coil. The sequence of 180° pulses causes a sequence of spin-echo signals, but with decreasing signal strength over the sequence of 180° pulses.
In gradient-echo detection, a sequence of gradient defocusing and focusing follows an excitation pulse. The application of the defocusing and refocusing magnetic gradient changes generates a detectable echo signal in a receiver coil, but with decreasing signal strength with increasing repetitions of magnetic gradient defocusing and refocusing, and increases in time required for magnetic gradient defocusing and refocusing.
Because the Larmor frequency is a function of position due to the gradient in the static magnetic field, NMR imaging is realized by changing the frequency of the oscillating magnetic nuclei and analyzing the resulting spin-echo or gradient-echo signals. However, in applications to medical imaging of the human body, there may be various medical implants whose magnetic susceptibility does not match that of the surrounding tissue. A mismatch in magnetic susceptibility will affect the magnetization, which may lead to imaging artifacts in NMR. For example, relatively small deviations in the magnetic field may result in the displacement of several voxels when performing NMR imaging. Furthermore, signal loss may occur due to dephasing. During the acquisition when the free induction signal is refocused, the average frequency of the protons should remain constant. But in areas where there are deviations in the magnetic field homogeneity, the received signal is reduced because of the loss in refocusing.
Acrylics used in dental work are manufactured to be radio-opaque, so that they may be imaged in an x-ray. However, various materials in the acrylics may have a susceptibility different from that of air or water, which may cause unwanted artifacts in an NMR image. As another example, various ceramics may be used in hip replacements and other medical implants, where the magnetic susceptibilities of such ceramics may not match the surrounding tissue susceptibility (e.g., the susceptibility of water).